1. Field of the Invention
This invention relates to an auto-focusing apparatus which obtains an evaluation value from a contrast component of a video signal and performs focus adjustment so that the evaluation value will be at a maximum.
2. Description of the Prior Art
In the existing picture-processing auto-focusing apparatus, auto-focusing is performed by driving a focus lens so that an evaluation value which is obtained by a contrast component of a video signal will be at a maximum, based on a concept that focalization is completed when the high-frequency component of the video signal is at a maximum (FIG. 1). More specifically, the direction and speed of the focus lens are controlled by the sign and scale of a change (.DELTA.y) in an evaluation value, when a focus lens is moved in a very small amount of length (.DELTA.x).
Which part in the screen will be an object of auto-focusing is determined by a frame on the screen to obtain the evaluation value (called hereafter "measurement frame"). Demerits vary with whether the measurement frame is large or small. For example, when the measurement frame is large, focusing is made at a background, if a contrast at the background is striking. This is what is called "background focusing" (FIG. 2B). On the other hand, when the measurement frame is small, an object may not be contained in the measurement frame (FIG. 2A), or an edge may not be contained (FIG. 2C). In FIGS. 2A, 2B, and 2C, reference character W shows a measurement frame and reference numeral 100 denotes a screen.
As mentioned above, an optimum measurement frame differs with contents of objects. It has therefore been difficult to determine an adequate size of the measurement frame.